There are a number of applications in which it is desirable to remove hazardous contaminants such as radon from a fluid such as air or water. Radon accumulation in homes, schools, and other buildings is recognized as a serious health problem. Radon is a radioactive noble gas produced in the ground by the natural decay of uranium and radium. The radon further decays to a chain of daughter products which associate with dust particles in the air. The daughter products emit harmful alpha radiation which, when inhaled, damages lung tissue. Tens of thousands of cases of lung cancer are estimated to arise each year from radon infiltration into homes.
While the majority of the source of radon in the air of buildings is due to soil gas, the potential also exists for radon to be released from well water that has radon dissolved in it. Many thousands of private wells and additional municipal and Federal water supplies may be affected. This waterborne radon is not a hazard when it is dissolved in the water, but it can become airborne when the water is used. A shower, faucet, or toilet can release the radon into the indoor air and can cause localized high concentrations of the gas. This radon gas then diffuses through the rest of the building and can cause an overall high level of radon.
For every 10,000 pCi/L of radon in household water, there will be an increase of about 1 pCi/L in the air inside a home. Levels of about 4 pCi/L in the air or a waterborne level of 40,000 pCi/L are above recommended levels set by the Environmental Protection Agency.
A fiberglass tank containing a bed of granular activated carbon (GAC) has been proven to be effective in removal of low concentrations of impurities in water. The GAC tank contains approximately 2.5 cubic feet of charcoal and is typically installed in the main water supply line after the pressure tank. The high removal efficiency is achieved by allowing the radon gas to decay in the charcoal bed, thus eliminating it as a source. The deca of radon in the beds, however, leads to the build-up of daughter products on the carbon. In particular, Pb-214 and Bi-214 accumulate, and when they decay, they give off gamma radiation. Prior research has shown that roughly one mRem per hour of gamma activity is given off for every 17,800 pCi/L. In some cases, gamma exposure rates of up to 72 mRem per hour have been documented. The potential for these high external radiation fields and the additional concern for lead build-up on the carbon, as well as concerns for disposing of the contaminated beds, discourages using a GAC tank.
Another filter system uses aeration to remove the radon from the water before it is used. However, aeration is performed at atmospheric pressure and therefore requires depressurization and then repressurization of the water supply. Aeration equipment is also relatively costly, occupies a considerable amount of space, and has numerous pieces of operating equipment which may require significant maintenance cost.